It is known that paper-like non-woven sheets are produced from various types of synthetic fibers, for example, polypropylene fibers, acrylic fibers and water-insoluble polyvinyl alcohol fibers. These non-woven sheets are produced by a wet sheet-forming process, a dry sheet-forming process or a spun bond process. Also, it is known that among the various sheet-forming processes, the wet paper-making process is the most beneficial for industrially producing the synthetic fiber non-woven sheet. However, the non-woven sheets produced from the above-mentioned types of synthetic fibers are unsatisfactory in mechanical strength, thermal resistance, touch and/or filtering property.
Recently, it was attempted to utilize polyester fibers to produce a paper-like non-woven sheet. However, the resultant paper-like polyester fiber sheet was still unsatisfactory in mechanical strength, and touch, softness, uniformity of quality and structure. Especially, it was found that, in the case of filtering material made from the polyester fiber sheet, a modification of the filtering material so as to increase the collecting effect of the filtering material causes the pressure loss of the filtering material to increase and causes the durability of the filtering material to be shortened. Also, the modification of the filtering material so as to decrease the pressure loss or to increase the durability of the filtering material, causes the collecting effect of the filtering material to be decreased.
Also, it was found that in the wet paper-making process, conventional polyester staple fibers were unevenly dispersed in water and the resultant sheet was frequently broken during the paper-making process. Furthermore, the resultant paper-like sheet was unsatisfactory in quality.
Accordingly, the conventional paper-like polyester fiber sheets made by the wet paper-making method were not successful in practical industrial production and use.
U.S. Pat. No. 2,836,576 and Japanese Patent Application Publication No. 49-8809(1974) disclose a polyester fiber sheet in which undrawn polyester fibers are used as a binding material for drawn polyester fibers. Also, Japanese Patent Application Publication No. 51-2542(1976) discloses a polyester fiber sheet in which undrawn polyester fibers are used for the purpose of enhancing the tear strength of the sheet. However, the above-mentioned polyester fiber sheets exhibited unsatisfactory mechanical strength, touch and appearance, and, therefore, were practically useless.